Currently the clean-energy technologies are experiencing a surge in popularity, this surge being driven by rising demands for high-output and fuel-efficient energy with reduced or no dependence on the petroleum industry.
Among the various clean-energy technologies, the electrochemical energy storage technologies—especially lithium-ion batteries—attract a lot of attention owing to their relatively low mass and high energy density. The lithium-ion battery (LIB) is widely used in consumer electronics such as cell phones, portable computers and cameras, where it has over 90% of the market and wherein the market value is expected to reach US$43 billion by 2020.
In addition to the mature market in consumer electronics, a key growing market for LIBs is in electric power storage grids and in the automotive and transportation industry, especially in electric vehicles. With the growing demand to reduce carbon dioxide emissions and dependence on fossil fuel energy and with the ever-increasing interest in sustainable ecologically-friendly high-efficiency energy systems, an electrochemical energy storage device such as the LIB provides a viable alternative. By 2015, the automotive LIB market value is expected to reach US$9 billion.
The most important requirements for automotive lithium ion battery are reasonable cost, high electrochemical performance (fast charging/discharging times), long service life (including reliability in abusive situations such as thermal or mechanical shocking) and high safety.
Conventional LIB design comprises an electrolyte, a lithium cobalt dioxide (LiCoO2) cathode and a graphite anode. During the discharge phase of the LIB, the intercalated lithium in graphite is released and migrates towards the cathode. At the same time, electrons flow to the cathode, generating electricity.
Traditionally, graphite is the most commonly used anode material in LIBs. However graphite-based LIBs exhibit several weaknesses, including:                at full-charged state, lithiated graphite electrodes are highly reactive;        thermal degradation of passivation films occurs at temperatures in the range of 100-150° C., resulting in thermal runaway which leads to a violent exothermal reaction or explosion;        low working voltage, close to that of metallic lithium.        
These weaknesses contribute to safety concerns.
While in terms of the cathode, lithium iron phosphate (LiFePO4) is the cathode material of choice for automotive applications, as anode graphite is not considered viable for electric vehicles, hence the interest in lithium titanate. Of the numerous lithium titanate compositions, Li4Ti5O12 (LTO) is a preferred electrode material—it is safe and good for high-rate and long-life automotive LIBs.
Graphite is a relatively inexpensive material and it has a very good capacity, but it suffers from relatively poor safety (due to formation of the so-called “solid electrolyte interface (SEI)”), short lifetime and slow charging/discharging characteristics (low performance). By comparison, lithium titanate (LTO) has a capacity of only 175 mAhg−1, a value 50% that of graphite, but advantageously has zero strain during charging/discharging phases (i.e. a volume change of only 0.2% during lithium ion intercalation) which leads to long service life and 100× shorter charging time than graphite, negligible to no Li-deposition when overcharging and no solid electrolyte interface (owing to its high and flat working voltage, 1.55V) which makes lithium titanate extremely safe (Table I). Moreover, the relatively small particle size of LTO is beneficial to the diffusion of lithium into the crystal structure. Consequently, lithium titanate-based LIBs are well-suited to the automotive industry and are in use in all kinds of vehicles, including electric vehicles (EV) and plug-in hybrid electric vehicles (PHEV).
TABLE 1Comparison of electrode material: Li4Ti5O12 (LTO) vs. GraphiteLi4Ti5O12 (LTO)GraphiteVolume Change0.2% (zero strain)12%Lithium Diffusion Coefficient10−8 cm2s−110−10-10−11 cm2s−1Working Voltage vs Li+/Li (V)1.55~0.1Solid Electrolyte Interface (SEI)none toforming ininconsiderable1st chargeTheoretical Capacity (mAh g−1)175372
As can be seen from the pseudo-binary phase diagram of the Li2O—TiO2 system (FIG. 1—prior art), the region of Li4Ti5O12 (LTO) is extremely narrow thus making the preparation of phase-pure LTO difficult. LTO is usually produced via formation (or use) of an intermediate Ti-oxide phase that is converted by thermal treatment to the final product. The existing routes to synthesize LTO include solid state, hydrothermal and sol-gel processes (graphically summarized in FIG. 2). For solid state synthesis, the quality of the products can be of concern. The titanium source compound (usually titanium dioxide, TiO2) and the lithium source compound (usually lithium carbonate, Li2CO3) are annealed at high temperature, usually over 750° C., so as to obtain relatively pure well-crystalline LTO, but this results in primary particle coarsening and inhomogeneous composites. The product obtained via solid-state synthesis must therefore be ground thoroughly, which may result in further impurities. Sol-gel processes can prepare high-quality nanostructured LTO, but the large amounts of organic solvent and chelating agent (which are expensive and highly polluting) as well as the necessary high temperature annealing step and the processes' relatively poor scale-up features impede the adoption of sol-gel processes for low-cost and high-volume production. Hydrothermal processes advantageously can more readily achieve nanosized particle products, however high-purity products are not easily produced and hydrothermal processes are generally more expensive than their solid state counterparts due to the high cost of precursor material such as titanium isopropoxide (TTIP), and the high-pressure equipment required.
Patent application WO 2010/052362 discloses a lithium titanate product, the formula of which is of the form LixTiyOz wherein when y is 1, the x:y molar ratio is 1.1-1.8, while the z:y molar ratio is 2.0-4.5. In addition, a process of preparing alkali metal titanate is described. In the process, an aqueous titanium-containing slurry is prepared and mixed with an alkali metal compound forming alkali metal titanate. The alkali metal compound is preferably an alkali metal hydroxide, preferably lithium hydroxide. The aqueous, titanium-containing slurry is comprised essentially of sodium titanate and is preferably prepared from titanyl sulphate, preferably prepared from an ilmenite concentrate by means of sulphuric acid and by thermal hydrolysis into titanium dioxide hydrate. The presence of sulphuric acid results in hazardous by-products necessitating proper safe disposal and hence additional cost.
Patent specification JP9309727 discloses a process for producing dense, flaky or plate-like lithium titanate by carrying out heat treatment of lithium titanate hydrate obtained by reacting a specific titanic acid compound with a lithium compound in water. A titanium compound is reacted with an ammonium compound in water to produce a titanic acid compound. The titanic acid compound is then reacted with a lithium compound in an aqueous solution of ammonium compound, and the reaction product is dried to give lithium titanate hydrate. The presence of ammonia presents technical problems such as the evaporation of ammonia when the pH rises above 7 and the nitrogen in the used solution which presents an environmental problem requiring further processing before safe disposal.
Liu et al. (Electrochim. Acta, 2012, 63, 100-104) discloses a microwave-assisted hydrothermal method for the synthesis of Li4Ti5O12. In Liu et al., hydrothermal treatment of a solution containing titanium isopropoxide as precursor, LiOH plus H2O2 is carried out at 130-170° C. to prepare an intermediate that is subsequently transformed to LTO by calcination at 550° C. One of the intermediates—the one produced at 130° C.—was lithium titanate hydrate (LTH). The use of a non-conventional energy intensive method as is microwave-assisted hydrothermal in combination with the use of a high cost organic titanium precursor (TIP) remain serious limiting factors to scale up and commercial feasibility.
The present invention provides a way to produce high purity nano-structured LTOs that is cost effective as well as more ecological as compared to conventional processes for producing LTOs. The LTO materials of the present invention are useful in a variety of applications including energy storage devices such as LIBs.